JP-11-193175-A2, corresponding to U.S. Pat. No. 688,677, and JP-2001-19269-A2 describe various sheet post-processing apparatuses which process stapling, punching, and folding of sheets.
In particular, a sheet bundle folding apparatus with movable push-in member described in both JP-11-193175-A2 and U.S. Pat. No. 688,677 includes a stick-out plate which follows a position of a fold on a sheet bundle when a pair of folding rollers holds the sheet bundle. The sheet bundle folding apparatus includes the pair of folding rollers, the stick-out plate, a pair of sliding-rollers, a groove, and a spring. The pair of sliding-rollers slides in the groove. The pair of sliding rollers is attached on the stick-out plate to support the stick-out plate. A diameter of one of the pair of sliding-rollers is smaller than a width of the groove.
A shaft of one of the sliding-rollers is pulled across the longitudinal direction of the groove by the spring connected with a chassis of the sheet bundle folding apparatus. The pair of sliding-rollers follows the stick-out plate advancing and pulling out. A pivot of a first end of the spring is stationary on the chassis, and a second end of the spring follows one of the pair of sliding-rollers. Therefore, the spring varies its posture (e.g., tilt angle from a direction perpendicular to a direction where the stick-out plate advances along) according to a position of the stick-out plate. The pivot causes an abrasion on both ends of the spring which is shaped as a hook or a ring. To avoid the abrasion, a bearing structure may be employed for the pivot. However the bearing structure is expensive.
Additionally, the tilt angle of the spring causes a reduction of an elemental force across the longitudinal direction of the groove. As a result, the stick-out plate changes position to push sheets to create fold on the sheets each time, and a fold on a sheet bundle changes each time. To avoid the abrasion, a strong spring may be employed. However the strong spring causes an undesirable side effect; namely, increasing a resistance force against advancing the stick-out plate along the longitudinal direction of the groove can result.
On the other hand, a recording paper after-treatment device for a picture image formation device described in JP-2001-19269-A2 includes a post processing tray for supporting a sheet bundle including sheets provided from the picture image formation device, a central folding roller pair for making a nip therebetween, and a central folding plate for pushing the sheet bundle into the nip to fold the sheet bundle.
The post processing tray bends to the nip to guide the sheet bundles smoothly. An upper one of the central folding roller pair is pushed upwards by the other folding roller in the pair and the sheet bundle by a thickness of the sheet bundle. However, the post processing tray does not move. That is, the nip varies its relative position against the post processing tray. Therefore, a fold on the sheet bundle varies its position according to its thickness. Moreover, the pressure for the sheet bundle by the central folding roller pair varies according to the thickness to make wrinkles.